1. Field of the Invention
The present invention relates to a fixed codebook searching apparatus and a fixed codebook searching method to be used at the time of coding by means of speech coding apparatus which carries out code excited linear prediction (CELP) of speech signals.
2. Description of the Related Art
Since the search processing of fixed codebook in a CELP-type speech coding apparatus generally accounts for the largest processing load among the speech coding processing, various configurations of the fixed codebook and searching methods of a fixed codebook have conventionally been developed.
Fixed codebooks using an algebraic codebook, which is broadly adopted in international standard codecs such as ITU-T Recommendation G.729 and G.723.1 or 3GPP standard AMR, or the like, is one of fixed codebooks that relatively reduce the processing load for the search (see Non-patent Documents 1 to 3, for instance). With these fixed codebooks, by making sparse the number of pulses generated from the algebraic codebook, the processing load required for fixed codebook search can be reduced. However, since there is a limit to the signal characteristics which can be represented by the sparse pulse excitation, there are cases that a problem occurs in the quality of coding. In order to address this problem, a technique has been proposed whereby a filter is applied in order to give characteristics to the pulse excitation generated from the algebraic codebook (see Non-Patent Document 4, for example).    Non-patent Document 1: ITU-T Recommendation G.729, “Coding of Speech at 8 kbit/s using Conjugate-structure Algebraic-Code-Excited Linear-Prediction (CS-ACELP)”, March 1996.    Non-patent Document 2: ITU-T Recommendation G.723.1, “Dual Rate Speech Coder for Multimedia Communications Transmitting at 5.3 and 6.3 kbit/s”, March 1996.    Non-patent Document 3: 3GPP TS 26.090, “AMR speech codec; Trans-coding functions” V4.0.0, March 2001.    Non-patent Document 4: R. Hagen et al., “Removal of sparse-excitation artifacts in CELP”, IEEE ICASSP '98, pp. 145 to 148, 1998.
However, in the case that the filter applied to the excitation pulse cannot be represented by a lower triangular Toeplitz matrix (for instance, in the case of a filter having values at negative times in cases such as that of a cyclical convolution processing as described in Non-patent Document 4), extra memory and computational loads are required for matrix operations.